20.5.06

BEFORE they went their separate evolutionary ways, the ancestors of chimpanzees and people got up to plenty of, well, monkey business. Moreover, this went on for about four million years. The most detailed analysis conducted of human and chimpanzee DNA reveals that after an initial separation from a common ancestor, between five and six million years ago, the species continued interbreeding. The implication is that speciation - the separation from a common ancestor - wasn't the simple process scientists previously believed. Instead, it happened over millions of years during which "episodes" of hybridisation took place before the final separation into two distinct species, US researchers claim in a paper published online by Nature. "For the first time, we're able to see the details written out in the DNA," said biologist Eric Lander, director of the Broad Institute at the Massachusetts Institute of Technology in Cambridge. "What they tell us at the least is that the human-chimp speciation was very unusual." According to Dr Lander and colleagues at Harvard University, they didn't expect to find evidence of human-chimp hybrids. "Hybridisation is commonly observed to play a role in speciation in plants, but evolutionary biologists do not generally view it as an important way to produce a new species in animals," said team leader Nick Patterson, a biostatistician at the Broad Institute. Geneticist David Reich, of Harvard Medical School, added: "That such evolutionary events have not been seen more often in animal species may simply be due to the fact that we have not been looking for them." While some experts in human evolution remain sceptical of some of the details, they are impressed nevertheless. "It's a totally cool and extremely clever analysis," said Harvard biological anthropologist Daniel Lieberman, who was not involved in the research. "My problem is imagining what it would be like to have a bipedal hominid and a chimpanzee viewing each other as appropriate mates, not to put it too crudely," he said. Previous studies comparing human and chimp DNA could only offer an estimate of how long ago the two species split by averaging the amount of divergence in their genes. Generally, those studies come up with a figure of about 7 million years. But thanks to the completion of the chimpanzee genome project in September, the team had about 800 times more data. That meant they were able to look at how specific sections of the genetic code evolved. For one thing, the new data suggest the human-chimp split was much closer to the present than the seven-million-years date that fossils and previous studies indicated - certainly no earlier than 6.3 million years ago, and more likely around 5.4 million. The data also show that the human-chimp split probably took four million years. That's because in some parts of the DNA sequence, the genetic difference between humans and chimps is so large that those genes must have been isolated from each other nearly 10 million years ago. But in other places, the human and chimp lines are so close that they appear to have still been swapping genetic material at least until 6.3 million years ago. One of those areas is the x-chromosome. Female chimps and humans have two x-chromosomes, while males have an x and a y. "The genes that are a barrier to speciation tend to be on the x-chromosome," team member Assistant Professor Reich said. But as interbreeding is known to place strong selective pressures on sex chromosomes, that would explain the discovery that the x-chromosome is some 1.2 million years younger than the rest of human chromosomes, the team suggested.